In recent years, an amorphous silicone-based electrophotographic photoreceptor having a layer made mainly of amorphous silica has been receiving attention as a light-sensitive material. The reasons for this are that amorphous silicone itself has a possibility of radically improving the life factors of the conventional electrophotographic photoreceptor and if it is applied to an electrophotographic photoreceptor, there is a possibility of an electrophotographic photoreceptor being obtained, said photoreceptor having electrically stable repeating characteristics, being of high hardness and thermally stable and thus having a long service life. From this point of view, various amorphous silicone-based electrophotographic photoreceptors have been proposed a described in JP-A Nos. 54-78135 and 54-86341 (The term "JP-A" as used herein means an "unexamined published Japanese patent application).
Among these, electrophotographic photoreceptors is an amorphous silicone electrophotographic photoreceptor having a so-called function separated type light-sensitive layer, i.e., a light-sensitive layer consisting of charge generating layer to generate a charge carrier upon irradiation of light and charge transporting layer in which the charge carrier generated in the charge generating layer can be injected with high efficiency and further the charge carrier is efficiently movable. As the charge transporting layer of the function separated type amorphous silicone electrophotographic photoreceptor, for example, an amorphous silicone film with a film thickness of about 5 to 100 .mu.m as obtained by decomposing a mixed gas of silane compound (e.g., silane or disilane) gas, carbon, oxygen or nitrogen-containing gas, and a small amount of Group III or V element-containing gas (e.g., phosphine or diborane) by glow discharging is used as described in JP-A No. 62-9355.
In general, in the electrophotographic photoreceptor divided into the charge transporting layer and the charge generating layer, the charge transporting layer with the largest film thickness among the light-sensitive layers is responsible for charging properties. However, charging properties of an electrophotographic photoreceptor using an charge-transporting layer of hydrogenated amorphous silicone film obtained by glow discharge decomposition of a silane compound as described above are such that the charge potential is about 30 V/.mu.m or less, and thus are not sufficiently satisfactory. Moreover, its dark decay rate is generally about 20%/sec or more, which is markedly high, although it varies depending on the conditions of use. For this reason, an electrophotographic photoreceptor using such an amorphous silicone based electric charge transporting layer is limited to a relatively high-speed system in application, or it needs a specified developing system because a sufficiently high charged potential cannot be obtained. To increase the charged potential, it suffices to increase the thickness of the electric charge transporting layer. For this increasing the layer thickness, however, it is necessary to lengthen the production time and moreover, in accordance with the usual process of production, the possibility of formation of film defects due to the formation of such a thick film is increased, resulting in a reduction of yield and a great increase in production costs.
In order to overcome the problems of the prior art as described above, the present inventors have proposed an electrophotographic photoreceptor using an aluminum oxide layer as the charge transporting layer in JP-A No. 63-63051. As a result of further investigations, it has been found that more preferable results can be obtained if the aluminum oxide film is produced by a specified method.